A rock falling off a cliff is different from the same rock lying on the ground below. A rubber band pulled taut is different from the same rubber band left slack. A glowing lightbulb is different from the same bulb when the electricity is switched off. It is the same rock, the same rubber band, the same lightbulb. The difference is one of energy.
Energy is one of the most basic ideas of science. All activity in the universe can be explained in terms of energy and matter. But the definition of energy is not at all simple since energy occurs in many different forms, and it is not always easy to tell how these forms are related to one another and what they have in common. One of the best-known definitions of energy is the classical definition used in physics: Energy is the ability to do work.
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Citation: "Energy." Britannica School, Encyclopædia Britannica, 8 May. 2018. school.eb.com/levels/middle/article/energy/274180. Accessed 28 Jan. 2019.